famouschinesec, just to confirm - you have two vouchers, which must be in two different names, correct?
As Bearvet says, On person needs to redeem the GAD voucher for the ticket first, just so they have a ticket. You can ask about doing the upgrade at that point - they may or may not do it. If not, you can do it any time afterwards at any ticket booth or GR location.
The other person needs to ALREADY HAVE a multi-day ticket in hand in order to get the Fastpass option.
As for where, you would do it at the Will Call window for the appropriate park. For Epcot, DHS and AK it will be one of the ticket booths, with a Will Call sign. For MK, there is a Will Call ticket booth at the TTC, but it can also be done at the main gate at MK - usually it is the Guest Relations windows to the right of the turnstiles, on busier days they may use the round building nearby - there will be a sign with an arrow pointing to it.